Light emitting diodes (LEDs) are considered attractive light sources for various applications such as such as traffic signals, displays, automobile headlights and taillights and conventional indoor lighting. LEDs are generally more reliable and efficient than other light sources, such as incandescent bulbs.
However, with LEDs only a small portion (about 2%) of light generated within the LED active layer can be extracted and utilized while the remaining part is absorbed within the LED structure itself. This is due to the difficulty for light to be extracted from LED semiconductor materials, which have a relatively high index of refraction. Typical LED semiconductors have index of refraction ranging from 2.2 to 3.8, which is high when compared to that of ambient air (about 1.0).
Many methods for increasing the LED efficiency (i.e., extracting more light from the LED active layer) have been reported. Shnitzer, et al. in “30% External Quantum Efficiency From Surface Textured, Thin Film Light Emitting Diodes”, Applied Physics Letters 63, 1993, pp. 2174-2176, propose a method of introducing random nanotexturing on the LED's surface. Since the introduced features are on the order of the wavelength of light, the light behavior becomes chaotic leading to enhanced LED efficiency.
Other methods introduce periodic or non-periodic patterns (rather than random texturing) on the order of light wavelength to the emitting surface or internal interfaces of the LED. Due to interference effects, more light is extracted from the LED active layer leading to enhanced efficiency. Examples of this method are discussed in U.S. Pat. No. 5,779,924 to Krames et al. and U.S. Pat. No. 6,831,302 B2 to Erchak et al.
Shnitzer, et al. in “Ultrahigh Spontaneous Emission Quantum Efficiency, 99.7% Internally and 72% Externally, From AlGaAs/GaAs/AlGaAs Double Heterostructures”, Applied Physics Letters 62, 1993, pp. 131-133, propose photon recycling for extracting more light from the LED. This method requires materials with extremely low optical loss and the use of a non-absorbing current spreading layer on the LED surface.
In another approach, Parkyn, Jr. et al. in U.S. Pat. No. 6,560,038 B1 propose the use of light pipes to extract more light from the LED. Krames, et al. in “High Power Truncated Inverted Pyramid (Alx Ga1-x)0.5 In0.5 P/GaP Light Emitting Diodes Exhibiting >50% External Quantum Efficiency,” Applied Physics Letters 75, 1999, teach the angling of the LED chip's side surfaces to create an inverted truncated pyramid and thus enhance the extraction efficiency.
Although they represent an improvement over plain LEDs, known methods for enhancing LED light extraction suffer from one or more of the following disadvantages: (a) added complexity to the LED fabrication process, (b) relatively high manufacturing cost, (c) and limited control over the spatial distribution of light in terms of angle and intensity.
Therefore, there is a need for simple, low cost and efficient light extraction system that provides control over spatial distribution of LED light in terms of intensity and angle.